1. Field of the Invention
The present invention relates to ink jet printing devices, and more particularly, to ink jet printing devices which utilize a deflection ribbon to cause selectively charged drops to be deflected from their nominal trajectory.
2. Prior Art
The present invention relates more particularly to ink jet recording devices of the type which use a deflection ribbon for deflecting charged drops into a catching device, such as are disclosed, for example, by Mathis U.S. Pat. No. 3,701,998 and Cassill U.S. Pat. No. 3,787,883. These devices generally incorporate a fluid supply reservoir which supplies a printing liquid to a plurality of orifices disposed in two parallel rows and from which are ejected uniform streams of droplets directed towards a printing medium. A charge plate is utilized to place a desired level of charge on selected ones of the droplets ejected from the orifices. All of the droplets then pass immediately adjacent a deflection ribbon which is oppositely charged from the droplets so that the selectively charged droplets are repelled from the ribbon into an adjacent catcher. The remaining uncharged droplets which are unaffected by the electrostatic field established by the deflection ribbon then impinge on the recording medium in the desired pattern.
One difficulty which has been discovered in the utilization of such printing devices is that due to the development of a fine mist which is generated by the printing liquid, the deflection ribbon and other parts of the ink jet printing device become sufficiently wet that shorts can occur between the charge plate and ribbon or between the catcher and ribbon. This situation is aggravated when the printing medium is a fiberous material such as paper, since fibers sometimes leave the paper and adhere to the ribbon, charge plate, or catchers and thus reduce the clearance therebetween and as a result increase the likelihood of shorts across these various elements.
Such shorts result in a loss of deflection voltage which in turn causes insufficient deflection of the selectively charged droplets in the electrostatic field and will thus cause such droplets to impinge upon the recording medium rather than be caught by the catcher, resulting in distortion in the intended sequence of printing.
Because of the possibility of electrical arcing between the deflection ribbon and adjacent parts of the printing device, it is necessary to keep the voltage on the deflection ribbon relatively low in order to inhibit such arcing. This in turn necessitates the use of a higher charge voltage on the charge ring in order to induce a higher charge on the selected droplets so that the low voltage on the deflection ribbon has sufficient effect on the charged droplets to deflect them into the catcher. It is in turn much more difficult to switch on and off a higher voltage than it is a relatively lower voltage because of the inherent time delay in the voltage dropping from its maximum value to substantially zero. This produces problems in the charge plate control circuitry and ultimately limits the speed at which the printing device can function.